The non-invasive monitoring of arterial oxygen saturation (SaO.sub.2) by pulse oximetry is used in many clinical applications. For example, SaO.sub.2 monitoring is performed during surgery, in critical care situations, for hypoxemia screening, in the emergency room, and in the field. The instruments are small and lightweight, making them ideal for neonatal, pediatric and ambulatory applications. Because this instrument is capable of providing continuous and safe measurements of blood oxygenation non-invasively, the pulse oximeter is widely recognized as one of the most important technological advances in bedside monitoring. In 1986, the American Society of Anesthesiologists recommended pulse oximetry as a standard of care for basic intraoperative monitoring, and in 1988, the Society for Critical Care Medicine recommended that this method be used for monitoring patients undergoing oxygen therapy. The mandatory or voluntary use of pulse oximetry by regulatory agencies and professional organizations is likely to continue.
Because pulse oximeters are small, easy-to-use and readily available, they have become widespread in the last decade. The high costs associated with health care make the use of non-invasive pulse oximetry very attractive as it permits effective oxygen monitoring without the expensive clinical laboratory analysis of blood samples.
Oxygen saturation measurements rely on the difference in optical absorbance of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO.sub.2), as shown in FIG. 1. HbO.sub.2 absorbs less light in the red region (ca. 660 nm) than does Hb, but absorbs more strongly in the infrared region (ca. 940 nm). If both wavelengths of light are used, their opposite change in light absorbed as HbO.sub.2 varies versus Hb produces a sensitive index of blood oxygen saturation. The "functional hemoglobin saturation" is defined as: EQU Functional SaO.sub.2 ={[HbO.sub.2 ]/[HbO.sub.2 +Hb]}.times.100% (1)
Pulse oximeters thus employ two discrete wavelengths of light, which are passed through a given tissue (typically a finger). The amount of transmitted light for each wavelength is detected and subtracted from the incident light to determine the amount absorbed. From the ratio (R/IR or "red/infrared") of the amount of light absorbed at each wavelength, the blood oxygen saturation is calculated from a predetermined algorithm. If these were the only conditions of the measurement, the calculated saturation value would in some degree reflect the mixture of arterial and venous blood flowing through the tissue. However, in pulse oximetry the time-variant photoplethysmographic signal, caused by increases in arterial blood volume due to cardiac contraction, is used to determine the arterial blood oxygen saturation (FIG. 2). The advantage of this method is that the oxygen saturation values of the relatively constant flow of arterial and venous blood, as well as the constant absorption of light by the tissue, are discarded.
The SaO.sub.2 values are derived by analyzing only the changes in absorbance caused by the pulsating arterial blood at a red wavelength (e.g., 660 nm), where the absorbance of HbO.sub.2 is less than that of Hb, and a second reference infrared wavelength (e.g., 940 nm), where the absorbance of HbO.sub.2 is slightly larger than Hb. Because the transmitted light intensities depend on the sensitivity of the detector and the individual intensities of the light sources (light-emitting diodes, or LEDs), and because tissue absorption can vary a great deal between individuals, a normalization procedure is commonly used. This normalization involves dividing the pulsatile (AC) component of the red and infrared photoplethysmograms (which is a result of the expansion and relaxation of the arterial blood) by the corresponding non-pulsatile (DC) component of the photoplethysmogram (which is due to the absorption of light by tissue, non-pulsatile arterial blood, and venous blood). This scaling process results in a normalized red/infrared ratio (R/IR) which is virtually independent of the incident light intensity. R/IR can thus be expressed as: EQU R/IR=[AC.sub.red /DC.sub.red ]/[AC.sub.ir /DC.sub.ir ] (2)
Pulse oximeters are calibrated empirically by correlating the measured ratio of normalized AC/DC signals from the red and infrared photoplethysmograms with blood SaO.sub.2 values obtained from a standard in vitro oximeter. A typical relationship between the normalized R/IR ratio and SaO.sub.2 is shown in FIG. 3. At approximately 85% SaO.sub.2, the amount of light absorbed by Hb and HbO.sub.2 is nearly the same, so the normalized amplitudes of the red and infrared signals are equal, and R/IR is 1. For properly functioning instruments, further calibration should not be required in the field because the optical properties of blood are fairly similar among different individuals.
Pulse oximeter probes consist of LEDs for two separate and discrete wavelengths (e.g., 660 and 940 nm) and a photodiode light detector. Three different light levels are measured by the photodiode: the red (660 nm) light level, the infrared (940 nm) light level, and the ambient light level. These three light sources are detected separately by a single photodiode by sequencing the red and infrared light sources on and off, allowing an interval when both are off in order to detect (and subtract out) ambient light. An example from the commercially available Ohmeda model 3700 pulse oximeter is shown in FIG. 4. Sequencing the red and infrared LEDs at a frequency that is an integer multiple of the power line frequency allows the system to operate synchronously with flickering room lights. For example, fluorescent lights generate a 120 Hz flicker on 60 Hz power. The sequencing avoids potential interference of light flickers on the photodiode that would distort or disguise the tiny pulse signals of arterial pulse flow. The light timing sequence shown in FIG. 4 cycles 480 times per second at 60 Hz power; 16 of the red-infrared-off sequences are used to calculate SaO.sub.2 every 0.033 second. These signals are used differently by different pulse oximeter manufacturers, as described below.
The response time of the instrument depends on the number of data points averaged before a final SaO.sub.2 reading is displayed. There are two basic approaches to this averaging, one of which relies on the time average of the peak-to-peak amplitudes of each pulse (FIG. 5A). This method depends on the patient's heart rate and is relatively slow as the signals are available for averaging only once every heartbeat. Another approach is to average a large number of step changes along the steep slopes of the photoplethysmogram (FIG. 5B). In this case, the response time in the instrument is shorter because there are many more data points between successive heartbeats; also, the accuracy and stability of the measured SaO.sub.2 values are usually improved by this approach. The accuracy of pulse oximeters has been extensively studied and has been found to be generally acceptable for a large number of clinical applications. Most manufacturers claim that their instruments are accurate to within .+-.2% in the SaO.sub.2 range of 70-100% and within .+-.3% for SaO.sub.2 values between 50 and 70%, with no specified accuracy below 50% saturation.
Most pulse oximeters offer other display features in addition to SaO.sub.2, such as the pulse rate and displays to indicate the pulse waveform and relative pulse amplitude. These help the user to partially assess the quality and reliability of the measurement. For instance, if the patient's actual heart rate does not agree with that displayed by the pulse oximeter, the displayed SaO.sub.2 value is brought into question. In addition, the shape and stability of the photoplethysmographic waveform often serves as an indication of possible motion artifacts.
Although pulse oximeters offer such advantageous features as described above, are now mandatory for all anesthesias and tens of thousand's of oximeters are in clinical use, doctors and hospitals have no way of knowing if the oximeters are working correctly. Until the present invention, there has not been a simple method or device for verifying oximeter operation despite a clear and pressing need. Manufacturers sometimes provide simple electronic simulators to test the electronic circuitry of their oximeters, but these do not test the performance of the optical sensor and therefore are inadequate. U.S. Pat. Nos. 4,968,137 and 5,166,517 are examples of prior art methods and devices for testing pulse oximeters.